The present invention relates to a brake-pressure control device for a road vehicle, having a hydraulic braking system, preferably a dual circuit braking system, and, more particularly to a brake pressure control device having a hydraulic multiple circuit braking system configured such that wheel braking is effected by operation of a brake pedal, a braking device comprising a master cylinder having output pressure chambers individually assigned to brakes and a pneumatic brake power assist unit having a vacuum chamber connected to an intake stub of a vehicle engine and a working chamber configured to be subjected, via a control element operable by the brake pedal, to a pressure higher than a pressure prevailing at the intake stub, sensor to detect a position of one of the brake pedal and an element motively coupled thereto and to generate electrical output signals characteristic of at least one of instantaneous position of the brake pedal and variations therein, and/or a force transmitter which generates output signals characteristic of a force with which a vehicle driver operates the brake pedal when braking, and an electronic control unit operatively associated with the sensor and/or the force transmitter such that, by processing signals from one of the sensor and the force transmitter, output signals are generated by the electronic control unit as drive signals for a brake-pressure control device so that, during a driving operation, a higher brake pressure than the brake pressure otherwise to be expected in accordance with the instantaneous pedal force can be coupled into individual wheel brakes, furthermore, driving of the brake-pressure control device for the purpose of increased brake-pressure deployment being triggered when at least one of a speed at which the brake pedal is operated for the purpose of a brake pressure build-up and a rate of variation of the force at which the vehicle driver operates the brake pedal is higher than a prescribed threshold value.
Provided in the foregoing type of braking system as a braking device is a master cylinder which is operable via a vacuum brake power assist unit of conventional configuration and which has output pressure chambers individually assigned in each case to the brake circuits. The pneumatic brake power assist unit has a vacuum chamber connected to the intake stub of the vehicle engine and a working chamber which is movably bounded with respect to said vacuum chamber by the working piston of the brake power assist unit and which can be subjected via a control element which performs the function of a 3/3 way control valve and is operable by the brake pedal to a pressure higher than the pressure prevailing at the output stub of the vehicle engine. As a result, the master cylinder can be operated by a force which is boosted with respect to the pure pedal force and which supports the brake pressure build-up in the wheel brakes. A displacement or position sensor is provided as an element of the brake-pressure control device to monitor the pedal position and generate electrical output signals characteristic of the instantaneous position of the brake pedal and thus also of variations therein. By the processing of these output signals there are obtained, via an electronic control unit, drive signals for a brake-pressure control device by way of which--in the event of drive--it is possible to couple into the wheel brakes a higher brake pressure than the brake pressure otherwise to be expected in accordance with the instantaneous pedal position, driving of the brake-pressure control device for the purpose of increased brake pressure deployment being triggered when the speed .PHI. at which the brake pedal is operated is higher than a prescribed threshold value .PHI..sub.s. Instead of, or in combination with, monitoring the pedal position and its rate of variation, it is also possible to monitor the force K.sub.p with which the driver operates the brake pedal and its rate K.sub.p of variation and to trigger the increased brake force deployment when the rate K.sub.p of variation overshoots a threshold value K.sub.s. It is assumed that the vehicle is equipped with an antilock system which during braking regulates the dynamically stable deceleration behavior of the vehicle.
DE 40 28 290 C1 discloses such a brake-pressure control device required for automatic control of a method for shortening the braking distance in critical driving situations. If during pedal operation the speed threshold value .PHI..sub.s exceeded, that known device builds up as high as possible a brake pressure in the shortest possible time as far as a brake pressure which produces full braking. The brake pressure is limited, however, by the dominant effect of the antilock system in accordance with the respectively prevailing road conditions.
The realization of this brake pressure control concept is possible, for example, by way of a brake-pressure control device which, by analogy with a traction control system (TCS) which operates in accordance with the principle of once again decelerating by automatic actuation of its wheel brake a driven vehicle wheel tending to spin, also performs this function for the wheel brakes of the non-driven vehicle wheels, when the threshold value .PHI..sub.s of the speed is exceeded at which the driver operates the brake pedal.
Such a realization of a brake-pressure control device acting on all the wheel brakes by analogy with a TCS, which would require blocking off the brake circuits from the braking device, has the disadvantage, however, that with the onset of the automatically controlled braking operation there is a loss of any reaction of the brake pressure coupled into the wheel brakes on the brake pedal. The brake pedal would then become "hard", that is no longer capable of further depression. Thus a pedal reaction occurs which, in the case of a fair number of drivers, will give the feeling that the braking system is not in order and will then lead to an inadequate reaction of such a driver; for example, retraction of the brake pedal and subsequent renewed operation thereof because the driver suspects, for example, that "additional" brake fluid must be "pumped" or fed into the master brake cylinder, and is thus led to take measures, momentary discontinuance of braking--which would certainly be inappropriate in a traffic situation requiring full braking.
In order to avoid this disadvantage, a brake-pressure control device is described in the pending German application P 41 02 497.4-21 which is suitable for carrying out the known method and provides buffer accumulators assigned to the brake circuits. The accumulators can be connected, in a valve-controlled manner, to the pressure outputs of the braking device by an output signal of the electronic control unit which activates the brake-pressure control device. The accumulators are blocked off from the brake circuits, however, and can be recharged against a smaller restoring force than a reaction force otherwise resulting from reaction of the brake pressure on the braking device and directed against the operating force, so that during braking, bracking fluid can be displaced into these buffer accumulators from the braking device. As a result, pedal travel becomes possible and, even after the onset of the automatic brake pressure control, a pedal feeling is retained which is essentially the same as in normally occurring braking, so that the driver cannot be irritated or distracted by the automatic brake pressure control.
In the brake-pressure control device in accordance with the aforementioned Application P 41 02 497.4-21, however, the technical outlay and space required in connection with the buffer accumulators and with the solenoid valves required therefor to be coupled to and blocked off from the braking device as needed are substantial and, to this extent, disadvantageous.
A vehicle is disclosed in DE-38 18 708 A1 having a hydraulic dual circuit braking system which is operable via a vacuum brake power assist unit of special design comprising a vacuum chamber (modulation chamber), a working chamber and a simulation chamber which in the brake release position is held connected in a communicating manner to the working chamber to obtain an antilock regulating function, i.e. reducing the brake pressure, by connecting the vacuum chamber which is normally at a lower pressure than the atmospheric pressure, in a valve-controlled manner to the ambient atmosphere. Consequently, a tappet acting on the push rod piston of the master cylinder of the braking system experiences a return movement as a result of which the pressure prevailing in the master cylinder is reduced. This return movement does not lead to a reaction on the brake pedal, since the input of the pedal operating force is performed via a brake valve whose housing displacement inside a simulator housing permanently remains, due to the effect of the pressures acting in the working chamber and in the simulation chamber, in a defined basic position, against the restoring force of simulation springs, which alone determine the "pedal feeling" which is communicated to the driver as feedback on the brake pressure corresponding to his wishes.
The known braking system described in DE-38 18 708 A1 can also be utilized to realize a traction control system by arranging the simulation chamber, which is normally connected to a vacuum source, to be blocked off from the vacuum source in a valve-controlled manner independently of pedal operation of the braking system. The working chamber is subjected to the ambient pressure, as a result of which the master cylinder is operable for the purpose of brake pressure build-up. Brake-pressure reduction phases which become necessary in the course of the traction control can be controlled in a manner analogous to the antilock control operation. However, if the known braking system operation is controlled by the brake pedal, the brake pressure build-up is always performed in a manner proportional to the operating force, so that raising the brake pressure above an amount input by the driver by his or her operating force is not possible by way of this braking system. The known braking system is therefore not suitable for realizing an automatically controlled braking operation in which, in an introductory phase of braking, a brake pressure deployment disproportionately increased with respect to the operating force is possible.
It is therefore, an object of the present invention to provide a brake-pressure control device which is suitable for carrying out the known method; facilitates, in conjunction with a nevertheless simple and cost-effective design, control in accordance with the method of an automatic braking operation in connection with a high vehicle deceleration; and thereby produces a pedal reaction largely corresponding to the accustomed pedal travel/brake force correlation.
The foregoing object has been achieved according to the present invention by providing the vacuum brake power assist unit includes a solenoid valve arrangement driven by one of the electrical output signals of the electronic control unit and movable from a basic position, in which a control duct of the control element, via which pressure compensation can be performed between the vacuum chamber and the working chamber of the vacuum brake power assist unit, is communicatingly connected to the vacuum chamber, whereas the working chamber of the vacuum brake power assist unit is blocked off from the outside atmosphere, into an excited functional position in which the control duct of the brake power assist unit is subjected to the ambient pressure via a flow path of the solenoid valve arrangement that is released in the functional position, whereas the control duct is blocked off from the vacuum chamber, the basic position of the solenoid valve arrangement being assigned to a non-operated state of the braking system and to the target braking operation thereof and the valve arrangement passing into its excited position when, during operation of the brake pedal, at least one of the threshold value of the rate of operation and the threshold value of the rate of variation of the operating force is overshot, and a valve is operatively associated to respond to the pressure in the control duct, is subjected to an opening direction to a relatively higher pressure in the control duct than in the working chamber and is otherwise blocked, and via which the control duct can be connected directly to the working chamber of the vacuum brake power assist unit.
A solenoid valve arrangement is provided as an auxiliary device on the vacuum brake power assist unit. When driven by the output signal of the electronic control unit which is characteristic of the rapid operation of the brake pedal, the solenoid valve arrangement can be switched over from a basic position, which is assigned to the non-operated state of the braking system and to the soft target braking operation thereof and in which a control duct of the control element of the brake power assist unit, via which pressure compensation can be performed between the vacuum chamber of the latter and its working chamber, is connected in a communicating manner to the vacuum chamber. The working chamber of the brake power assist unit is blocked off from the outside atmosphere, into an excited functional position which is assigned to the automatically controlled full braking and in which the control duct of the brake power assist unit is subjected to the ambient pressure via a flow duct of the solenoid valve arrangement that is released in this functional position, whereas the control duct is blocked off from the vacuum chamber of the brake power assist unit. By way of this solenoid valve arrangement, the accustomed pedal feedback is obtained due to the fact that the vacuum brake power assist unit is utilized during automatically controlled full braking in order to operate the master cylinder, with a drastically increased boost factor, at least to the extent that the brake pedal can at least follow up the brake power assist unit piston independently of which force has to be expended for this purpose by the driver. Thus, a pedal travel/vehicle deceleration correlation of an accustomed type is retained and the driver cannot be irritated or confused.
A valve which responds to the pressure in the control duct is pressurized in the opening direction by pressure in the control duct which is relatively higher than in the working chamber, is otherwise blocked and can directly connect the control duct to the working chamber. It is possible for this valve to be constructed as a check valve, and in a preferred configuration as a simple flutter valve, which sealingly covers the discharge opening of a transverse duct, which originates from the control duct and discharges into the working chamber, as long as the pressure in the working chamber is higher than in the control duct and, in a special configuration, is formed by an elastomeric lip which, subjected to moderate biasing covers the discharge opening of the transverse duct on the drive side. By way of this valve, it is possible for the control duct of the control element of the brake power assist unit to be connected directly to the working chamber thereof even if the control element, which in accordance with its function is a 3/3 way control valve having two alternative flow positions and one blocked position, is in the blocked position, which is adopted during normal braking when the driver no longer depresses the brake pedal upon the achievement of a desired vehicle deceleration. A desired rapid response of braking with a high boost factor at the onset is achieved by virtue of this valve, especially at the start of automatically controlled braking.
The brake-pressure control device according to the present invention can be realized by two 2/2 way solenoid valves, one of which provides in its excited position a direct connection between the working chamber and the outside atmosphere and is otherwise blocked, and the other of which provides in its basic position the connection between the control duct and the vacuum chamber and interrupts the connection in its excited position. It is possible for the first of these two valves to be arranged "outside" the brake power assist unit, and for the second to be integrated into the control element housing, which can be moved together with the working piston of the brake power assist unit.
From the foregoing, it should now be apparent that the technical outlay to be made for the brake-pressure control device according to the present invention corresponds at most to that which is necessary in the case of the brake-pressure control device according to the aforementioned German Patent Application P 41 02 497.4-21 merely for the connection control of the buffer accumulators further provided there, which control device is realized by 3/3 way solenoid valves.
Although flexible electrical supply lines and electrically insulating housing bushings are necessary for a solenoid valve integrated into the movable control element housing, the additional outlay in this regard is insubstantial compared with realizing the brake-pressure control device as provided in accordance with the German Patent Application P 41 02 497.4-21.
The only problem that could arise is the spatial arrangement of a valve, integrated into the control element, "inside" the brake power assist unit, which to this extent would require a design modification. However, outlay in this regard can be largely avoided, at least reduced, by leading the control duct to the outside via a flexible line and a bushing of the housing and arranging outside the vacuum chamber of the brake power assist unit a solenoid valve which in its basic position connects the bushing to a second bushing leading back into the vacuum chamber, and in its excited position adopted during driving by way of an output signal of the control device blocks off the first bushing from the second and in return connects the latter to the space outside the brake power assist unit, which is at atmospheric pressure. In such a configuration of the solenoid valve arrangement as a 3/2 way valve and arrangement of the latter "outside" the vacuum chamber of the brake power assist unit, only one valve is required, whose control current supply can also be performed via fixed electrical lines. When compared with a conventional vacuum brake power assist unit, the supplement, required to match the vacuum brake power assist unit to the brake-pressure control device according to the invention, of a flexible aerial line leading from the control duct to a first bushing of the vacuum housing, and of a second bushing, between which the brake-pressure control valve arranged outside the vacuum housing is inserted, entails only a small extra outlay and can therefore be realized cost-effectively.
It is especially advantageous when the brake-pressure control valve is constructed as a 3/3 way valve which by means of control signals of defined different control current strengths can be moved into its functional position II, which connects the control current duct to the outside atmosphere, and into a functional position I which blocks the control duct off from both bushings. It is possible to utilize the blocked position especially advantageously to control brake-pressure reduction phases in the course of which the brake-pressure control valve is switched over repeatedly between its basic position and the blocked position.
If, as provided in a preferred embodiment of the brake-pressure control device, the working chamber of the brake power assist unit can be subjected to a pressure higher than the atmospheric ambient pressure, the higher pressure being between approximately 1.5 and 2.5 (and preferably approximately 2 bars), it is possible to achieve increased braking forces during automatically controlled braking. An increase in the control point of the brake power assist unit is therefore achieved even during normal braking, when in this case the pump is activated. That is, it is possible to achieve relatively high braking forces via operating forces that are still relatively low.
In a simple embodiment that is advantageous in terms of circuitry, the pressure outlet of the compressed-air pump provided for the provision of the higher pressure is connected to the input, connected to the working chamber of the brake power assist unit during full braking, of the brake-pressure control valve via a first check valve which in the opening direction is subjected to relatively higher pressure at the pressure outlet of the pump than in the working chamber and is otherwise blocked. A second check valve is connected to the pressure inlet of the brake-pressure control valve and, in the event of relatively higher pressure at the pressure inlet of the brake-pressure control valve, is blocked and is otherwise open, so that the atmospheric pressure can be coupled into the working chamber of the brake power assist unit via the second check valve should the pump have failed.